Known portable electrostatic spray devices often suffer from poor, inconsistent spray quality when the charge-to-mass ratio of the product varies outside of a predetermined range. This may occur during transient conditions such as start-up and shut-down, or during steady state conditions such as when environmental conditions vary the load seen by the electrostatic spray device. In start-up conditions, for example, if the electrostatic spray device is allowed to begin spraying before the power supply circuit has fully charged the electrode to a desired potential, then the charge-to-mass ratio of the resulting spray may be below a desired level and may result in a poor quality spray exhibiting larger than desired droplet sizes and uneven spray patterns. Alternatively, after the electrostatic spray device has been turned off, charge stored in capacitive elements of the device may still be present and result in an after-spray condition until the charge in the capacitive elements has dissipated enough to stop a continuing flow of product from the nozzle of the electrostatic spray device. Further, during operation changes in environmental conditions such as humidity may significantly change the load seen by the high voltage power supply. Changes in the load will also affect the charge-to-mass ratio of the product and will alter the characteristics of the product spray.
U.S. Pat. No. 4,549,243 issued to Owen (the “Owen reference”) describes an electrostatic spraying apparatus that can be held in the human hand for applications such as graphic work where it is desired that the area to which the spray is applied can be precisely controlled (Col 1,11 5-9). A feature of the device disclosed in the Owen reference is that provisions may be made with said device for varying the potential applied to the nozzle, for example by varying the generator output, e.g. the frequency of production of high voltage pulses and/or their magnitude. The Owen reference discloses that this is advantageous since it enables fine, narrow, sprays to be produced (Col. 6, 11 37-42). Although the Owen reference does recognize a benefit for changing the output of the high voltage generator, the reference does not disclose sensing a spray load and adjusting the output of a high voltage power supply in response to a changing spray load. Nor does the Owen reference disclose providing user adjustable flow rates or for synchronizing the output of the high voltage power supply with the product flow rate to consistently obtain an optimal charge-to-mass ratio.
U.S. Pat. No. 5,121,884 issued to Noakes (the “Noakes reference”) presents an electrostatic sprayer designed such that potential surface leakage paths along which current may leak from the HT generator are sufficiently long to allow the use of a generator having a smaller than conventional maximum current output (Abstract). The benefit of reducing the current output required from the generator enables it to be built less expensively (Col 1, 11 12-14). Further, the Noakes reference identifies that the majority of the current supplied by the high voltage generator is surface leakage and unwanted corona discharge, only a portion being current actually used to charge the spray (Col 1, 11 33-37). The solution set forth by Noakes is to limit the surface leakage paths and to account for the leakage current in the current produced by the HT generator. An inherent problem with predicting the losses from the HT generator arises when operating a device in varying atmospheric conditions. With a change in atmospheric conditions (e.g. increased humidity) loses associated with corona discharge and surface leakage can either increase or decrease. To ensure that a particular device is capable of operation in a variety of atmospheric conditions, the device would need to be designed to function in the worst possible atmospheric condition (i.e. atmospheric condition corresponding with the highest corona discharge or surface leakage current). This would require operating a power supply for the worst case atmospheric condition thereby generating a significant amount of extra energy in atmospheric conditions that are not the worst case atmospheric conditions. Operating the power supply in this manner leads to an excess drain on battery power and increasing the possibility of charge build-up within the device leading to increased shock potential.